warren



March 9, 1965 R. w. WARREN 3,172,734

WATER COOLED CRUCIBLE FOR ZONE REFINING Filed Jan. 10, 1958 2Sheets-Sheet l 3 I FIG. 3. l I 2 I 0 O -o v o 4 F/G.

F 6 A 0 00 oo A "am e y March 9, 1965 R. w. WARREN 3,172,734

WATER COOLED CRUCIBLE FOR ZONE REFINING Filed Jan. 10, 1958 2Sheets-Sheet 2 72. W. Warren y Kb I Atlor ney United States Patent3,172,734 WATER COOLED CRUCIBLE FOR ZONE REFINING Reginald WalterWarren, London, England, assiguor to International Standard ElectricCorporation, New York, N.Y., a corporation of Delaware Filed Jan. 10,1958, Ser. No. 708,100 Claims priority, application Great Britain, Mar.7, 1957, 7,540/57 4 Claims. (Cl. 23-273) This invention relates to amethod and apparatus for processing fusible material. The invention isparticularly applicable to the processing of semiconductor material suchas germanium or silicon.

The processing of semi-conductor material involves melting the material.For example, zone refining of germanium or silicon is carried out bypassing a molten zone of the material from one end of a rod to another.It may be required to consolidate germanium or silicon in the form ofsmall lumps or powder into a coherent rod by melting the lumps or powderand casting the material into the form of a rod so that the process ofzone refining may be carried out on the rod.

In any process involving melting of semi-conductor material care must beexercised in the choice of the material for the crucible in whichmelting takes place. This is for the reason that very small quantitiesof impurities in semiconductor material cause large changes in theproperties thereof, and hence it is important that no impurities shouldbe introduced into the material from the crucible. Silicon, inparticular, is very liable to contamination when in the moltencondition. Hitherto crucibles for the treatment of germanium or siliconin the molten condition have been made of purified graphite or silica.It is, however, very difficult to ensure that graphite or quartzcrucibles are entirely free from impurities.

According to the present invention a crucible for processing fusiblematerial is made of metal of high electrical and thermal conductivityand in the form of a hollow walled vessel, fluid being circulatedthrough the hollow walls for cooling purposes. A work coil for inducingeddy currents surrounds at least a portion of said crucible and thecurrent through said coil is made of sufficient magnitude to melt thesaid material. 5

A molten zone may be caused to traverse the length of the crucible. Inthis way silicon or germanium in the form of granules or powder may beconsolidated into the form of a rod. A rod of silicon or germanium maybe refined in a similar way care being taken to observe the limitationsin the width of the zone for impurities to be driven to the ends of therod. The invention has been found to be highly successful in thetreatment of silicon. It has been found that, although silicon veryreadily absorbs impurities when in the molten condition, nocontamination of silicon occurs in the practice of the presentinvention.

The metals copper, silver and gold are chosen for the material of thecrucible because each of these metals has high electrical and thermalconductivity and does not react with the material being treated; silveris preferred because its electrical and also thermal conductivities arethe highest of the three and because it can be most readily polished toreflect heat optically into the melt. 65

One way of making the crucible is to start with a hol low cylinder ofsilver and to beat it into the shape of a boat. Practical tests withsilver have shown that molten silicon does not wet silver and does notpick up impurities therefrom. Copper and gold may, however, be used.

The invention will be better understood from the fol- 'ice lowingdescription of embodiments thereof taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a side view of one embodiment of apparatus according to theinvention.

FIG. 2 is a cross sectional view on the line 11 of FIG. 1, and inconjunction with the accompanying drawings in which:

FIG. 3 is a plan view of another embodiment, with part shown in section.

FIG. 4 is a cross sectional view of the line AA of FIG. 3.

FIG. 5 is a diagrammatic view illustrating one manner of operation ofthe apparatus.

Referring to the drawings and first to FIGS. 1 and 2, a crucible C ofsilver has in cross-section the form of a hollow half torus, throughwhich cooling water F is circulated by pipes D and E. Silicon B to bezone refined is contained within the crucible C. High frequency heatingcoils A are placed around the crucible in the places shown in FIG. 1.

As silicon is of very high resistivity when at room temperature asusceptor (not shown) is initially placed in a position adjacent to oneof the coils A and heats the silicon B by radiant heat to a temperaturesuflicient to reduce its resistivity to an extent to allow eddy currentsto be induced therein. This preheater is then removed. The current inthe heating coil A may be adjusted so that the electromagnetic fieldresulting therefrom and from currents induced in the metal of thecrucible C react with the currents induced in a molten zone of thematerial B in such manner as to raise such molten zone away from contactwith the metal of the crucible. For this purpose the crucible should beso located with respect to the heating coil that the silicon lies belowthe centre of the coil and the reaction between the electromagneticfield of coil A and the field clue to currents induced in the wall ofthe crucible immediately adjacent to the silicon with the field inducedin the molten zone of silicon lifts that zone upwards in the drawing.

This arrangement leads to economy in the power needed by reducing heatloss from the molten zone to the metal of the crucible.

Preferably the heat treatment is carried out in an atmosphere of aprotective gas such as argon and the crucible C is therefore containedin a chamber in which such protective atmosphere can be maintained.

In the apparatus shown in FIGS. 3 and 4 the crucible 1 is formed from ahollow cylinder of silver having copper rings 2 welded on its ends. Thecopper rings 2 are closed at one end of each except for aperturescommunicating with copper pipes 3. The dimensions of one tube 1 whichhas been used successfully are: length between copper end pieces 15inches, diameter 1% inches. The crucible is formed by pressing into theshape shown in the drawings the maximum depth of the depression formedbeing a little over half an inch. In operation cooling water iscirculated through the hollow space below the depression. An inductionheater 4 consisting of hollow copper tubes wound in a coil surrounds apart of the crucible 1. Cooling liquid may then be circulated throughthese hollow copper tubes. It is convenient to place the crucible 1within a tube 5 made of silica in order that the material in thecrucible can be surrounded by a protective atmosphere, the inductionheater 4 being exterior to the tube 5. The crucible 1 is then pushed orpulled, by means described later, so that the full length of thecrucible 1 passes the coil 4.

The apparatus shown in FIGS. 3 and 4 has been found to be particularlysuitable in the zone refining of silicon. As silicon is of extremelyhigh resistance when cold it is difficult to induce currents therein.Initially therefore susceptor rings 6 are placed in the neighbourhood ofcoil 3 4 and are heated by induction from the: coil 4. Radiant heat fromthe rings 6 then raises the temperature of the silicon sufliciently tolower its resistance and sufiicient eddy currents are then inducedtherein to melt the silicon in a limited zone immediately within thecoil 4. The susceptor rings 6 are then moved away.

The temperature gradient on either side of the limited molten zone isvery sharp since the silicon is cooled by the circulation of waterthrough the hollow crucible Walls. The temperature gradient does,however, permit of the molten zone being progressively traversed along arod of silicon lying in the crucible, the material immediately ahead ofthe molten zone in the direction of movement being of sufiiciently hightemperature to have currents induced therein when this material reachesthe centre of the coil.

One means for causing the traversal of the molten zone is showndiagrammatically in FIG. 5.

In this figure silicon 7 is shown contained within a crucible 1 of thekind above described. The copper pipes 3 pass through the center oftubular supports 8 secured to end plates 9. A silica tube surrounds thecrucible 1 and is secured to the end plates 9 in gas tight manner. Endplates 9 are mounted on a platform 10 which is in turn supported onwheels 11 running on a track 12. Depending from platform 10 is a nut 13engaged by a lead screw 14 rotatable through gearing 15 from a motor 16.Flexible hose 17 serves to circulate cooling water via copper pipes 3through the hollow Walls of crucible 1.

A protective gas, such as argon is circulated through the silca tube 5by flexible hose 18. By adjusting the speed of the motor 16 the platform10 carrying the crucible 1 within the protective gas atmosphere may betraversed past a stationary heating coil 4 to carry out the process ofzone refining at any speed found necessary or desirable and by reversingthe direction of rotation of the motor 16 passes of the silicon 7 may begiven in alternate directions.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What I claim is:

1. Apparatus for processing fusible material comprising an openelongated hollow walled vessel of a substantially pure metal of highelectrical and thermal conductivity which is substantially non-reactivewith the material being treated, means for circulating cooling fluidthrough the hollow walls of said crucible, a heating coil surrounding aportion of said vessel, means for passing through said coil alternatingcurrent of such value as to melt a limited zone of said fusible materialand means for causing relative movement between said coil and saidcrucible to melt successive limited zones of said material.

2. Apparatus for zone refining an elongated body of semi-conductormaterial comprising an open elongated hollow walled crucible forcontaining said body, said crucible being made of a substantially puremetal of high electrical and thermal conductivity which is substantiallynon-reactive with said semi-conductor material, means for circulatingcooling fluid through the hollow walls of said crucible, an inductionheating coil surrounding a portion of said crucible, means for passingalternating current through said coil of such frequency and power as toinduce suiiicient current in a limited zone of said body as to melt thematerial in said zone and means for causing such relativ movementbetween said body and said coil that said molten zone traverses at leasta portion of the length of said body.

3. Apparatus as claimed in claim 2 in which said coil is placed aroundthe crucible with the center above the center of said material so thatthe electromagnetic fields of the currents in said coil and thoseinduced in the metal of the crucible tend ot lift the material in themolten zone out of contact with said crucible.

4. Apparatus for processing a fusible material comprising a crucible ofsubstantially pure metal from the group consisting of copper, silver andgold, for containing said fusible material in contact therewith saidcrucible being in the form of a hollow walled vessel, means forcirculating a cooling fluid through said hollow walls to maintain saidcrucible below its melting point and below the melting point of saidfusible material and means for inducing currents in said crucible and inat least a part of the fusible material contained therein suflicient tomelt said fusible material through a cross sectional zone.

References Cited by the Examiner UNITED STATES PATENTS 2,686,865 8/54Kelly 23273 2,719,799 10/55 Christian 148l.6 2,768,074 10/56 Staugger23223.5 2,785,058 3/57 Buehler 23-301 2,836,412 5/58 Krieger 266--392,870,309 1/59 Capita 23301 2,872,299 2/59 Celmer 23-301 2,880,117 3/59Hamlet 2330l 2,897,329 7/59 Matare 23-301 OTHER REFERENCES Pfann:Transistor Technology, Bell Tel. Lab. Inc. and Western Electric Sp.,1952, pp. 44 to 54.

NORMAN YUDKOFF, Primary Examiner.

MAURICE A. BRINDISI, GEORGE D. MITCHELL,

Examiners.

1. APPARATUS FOR PROCESSING FUSIBLE MATERIAL COMPRISING AN OPENELONGATED HOLLOW WALLED VESSEL OF SUBSTANTIALLY PURE METAL OF HIGHELECTRICAL AND THERMAL CONDUCTIVITY WHICH IS SUBSTANTIALLY NON-REACTIVEWITH THE MATERIAL BEING TREATED, MEANS FOR CIRCULATING COLLING FLUIDTHROUGH THE HOLLOW WALLS OF SAID CRUCIBLE, A HEATING COIL SURROUNDING APORTION OF SAID VESSEL, MEANS FOR PASSING THROUGH SAID COIL ALTERNATINGCURRENT OF SUCH VALUE AS TO MELT A LIMITED ZONE OF SAID FUSIBLE MATERIALAND MEANS FOR CAUSING RELATIVE MOVEMENT BETWEEN SAID COIL AND SAIDCRUCIBLE TO MELT SUCCESSIVE LIMITED ZONES OF SAID MATERIAL.